At present, the KrF excimer laser light beam (wavelength: 248 nm) is principally used as the exposure light beam for the projection exposure apparatus based on, for example, the stepper type or the step-and-scan system to be used, for example, when a semiconductor integrated circuit is produced. Further, the use of the ArF excimer laser light beam (wavelength: 193 nm) will be started in near future as well. Investigation is also made for the development of an exposure apparatus which uses the fluorine laser beam (F2 laser beam) having a wavelength of 157 nm as the exposure light beam, in order to respond to the advance of finer semiconductor elements or the like.
However, also in the case of the vacuum ultraviolet region in which the wavelength is substantially not more than about 200 nm, and in the case of the light beam having a wavelength of not more than about 180 nm as in the F2 laser beam, the light is absorbed in an extremely large amount, for example, by molecules of oxygen, hydrogen, carbon dioxide gas, organic matters, and halides (hereinafter referred to as “light-absorbing substance”) which are abundantly contained in the ordinary air. Therefore, in order to obtain a desired illuminance (light amount), for example, on a wafer as a substrate to be exposed, it is necessary to sufficiently decrease the concentration of the light-absorbing substance so that the transmittance of the optical path for the exposure light beam is increased. In the case of oxygen, the amount of absorption is increased as the wavelength is decreased from about 200 nm. Therefore, in an exposure apparatus which uses the vacuum ultraviolet light as the exposure light beam, it is necessary to fill the optical path for the exposure light beam with a purge gas such as high purity nitrogen gas in which the light-absorbing substance as described above is scarcely contained.
When it is intended that the purge gas, in which the amount of the light-absorbing substance is greatly reduced, is supplied to the optical path for the exposure light beam in the exposure apparatus as described above, it is easy to provide a tightly enclosed structure, for example, for the illumination optical system and the projection optical system wherein it is relatively easy to maintain a high purity of the purge gas. On the other hand, in the case of the wafer stage section and the reticle stage section, some movable units exist, and it is necessary to exchange the wafer and the reticle as the mask. Therefore, it is necessary that the wafer stage section and the reticle stage section are capable of making any access to the outside, in which it is not easy to maintain a high purity of the purge gas.
Especially, in order to realize a high throughput, it is necessary for the wafer stage to frequently make access to the outside. Further, the photoresist, which is applied onto the wafer, generates the release gas (out gas) containing organic matters and halides which are included in the light-absorbing substance as described above and which absorb a large amount of the F2 laser beam. Further, the light-absorbing substance such as oil is continuously generated, for example, from wirings and movable members including, for example, stepping motors and linear motors which are installed in the vicinity of the wafer stage in many cases. Because there are several difficulties as described above, it has been difficult to fill the space (hereinafter referred to as “working distance region”) between the wafer and the tip of the projection optical system with the high purity purge gas.
In relation to the above, it is also conceived that the wafer stage is covered with walls and the internal space thereof (wafer chamber) is filled with the purge gas, as a method for purging the surroundings of the wafer stage including the working distance region with the high purity purge gas. However, in the case of this method, the following procedure is required. That is, a preparatory chamber, which is provided with double-entry doors, is provided between the outside and the wafer chamber for putting in and out the wafer. The wafer is loaded on the wafer stage after the air around the wafer is substituted with the purge gas in the preparatory chamber. Such a procedure involves the following inconvenience. That is, the mechanism is complicated, and the apparatus is expensive. Further, a long period of time is required to perform the gas substitution, and the throughput is lowered.
Usually, the position of the wafer stage is measured by a laser interferometer. However, it is feared that the measured value obtained by the laser interferometer may be varied due to any fluctuation of the optical path for the laser beam. In relation to the above, for example, when the wafer stage is moved during the process in which the high purity purge gas is supplied to the surroundings of the wafer stage including the working distance region, then the state of the flow of the purge gas is changed, and the fluctuation of the optical path for the laser beam takes place. As a result, it is feared that the positioning accuracy of the wafer stage may be decreased. The amount of decrease of the positioning accuracy is extremely small. However, in view of the fact that the degree of integration of the semiconductor element or the like will be further improved in future, it is desirable to suppress the amount of decrease of the positioning accuracy as described above as well.
Taking the foregoing points into consideration, a first object of the present invention is to provide an exposure method which makes it possible to maintain a high intensity of an exposure light beam on a substrate as an exposure objective.
A second object of the present invention is to provide an exposure method which makes it possible to easily supply a gas through which an exposure light beam is transmitted, to the space between a projection optical system and a substrate as an exposure objective.
A third object of the present invention is to provide an exposure method which makes it possible to avoid any decrease in positioning accuracy of a stage for positioning a substrate as an exposure objective when a gas, through which an exposure light beam is transmitted, is supplied to the space between a projection optical system and the substrate as the exposure objective.
A fourth object of the present invention is to provide an exposure apparatus which makes it possible to carry out the exposure method as described above, and a method for producing a device, which makes it possible to produce the device with a high throughput by using the exposure method as described above.